Footnotes[1] are[2] cool[3].
[1] A footnote is an additional piece of text which is referenced at a particular place within a main text. The footnote may add one or more of the following to the content of the main text at that point:

Additional explanation which some readers may require to fully understand the main text. For example, an expansion of an abbreviation, or a definition of a jargon term when it is first introduced.

Additional context for the point being made in the main text, to aid in interpretation and understanding.

A reference to an external work on which the main text is based.

A comment by the author about the main text, which is aside to the main narrative or argument being made.

Other meta-level comments on the main text. For example a note on the translation of a foreign language term.

An aside - an additional piece of information which is perhaps interesting and related to the main text, but which is not necessary to follow the flow of the main text.[4]

A footnote can be read as a sort of parenthetical comment, and could even be reformatted that way, but pulling it out as a footnote is less interruptive of the main text.

Some publications place the text of footnotes at the bottom[5] of the page on which they are referenced. Other publications collect all of the footnotes for a chapter and stick them all at the end of the chapter, while others collect them for the entire book and stick them all at the end of the book (in which case they may instead be called endnotes). Personally, I much prefer footnotes at the bottom of the page, because this makes it much easier to refer to the note quickly and see if it is something you want to read while you're in the right context. Footnotes/Endnotes at the end of a chapter or book force you to constantly flip back and forth between pages to see if you might be missing something important or interesting.

[2] The word "are" is used here as the third person plural present tense inflection of the English verb "be". "Are" also functions as the first and second person plural present tense inflections: "we are" (first person), "you are" (second person), and "they are" (third person); as well as the second person singular inflection: "you are". This may be easier to show in a table:

I am in Sydney.

Person

Singular

Plural

First

I am

We are

Second

You are

You are

Third

He/She/It is

They are

The verb "to be" is the most irregular verb in the English language. Most verbs follow the incredibly simple regular pattern, for example, "to walk":

Person

Singular

Plural

First

I walk

We walk

Second

You walk

You walk

Third

He/She/It walks

They walk

The verb "to be" is also the most common verb in the English language. It is what's called a copula, which is a kind of sentence binding word that is used in many cases as a sort of neutral syntactic glue when no other verb quickly comes to mind. (Notice that "is", the third person singular version of "to be", is used four times in the previous sentence alone.)

This usage as a copula annoys some people so much that they have proposed or used a version of English which eschews the verb "to be" (and all its inflections) altogether. For example, the sentence in the above paragraph ("It's what's called a copula...") with four uses of "is" in it could be rewritten as follows:

It plays the part of a copula, a syntactic element defined by the property of binding sentences together like a sort of syntactic glue when no other verb quickly comes to mind.

We are in a business meeting.

It takes quite a bit of effort to write like this, because the verb "to be" represents the go-to verb in most cases when constructing an English sentence. Try it, if you don't believe me. Remember that besides the present tense versions "be", "am", "is", are", there also exist the past tense forms "was", "were", the past participle "been", and compound forms like "will be", "had been", "will have been", and so on. You need to avoid all of those.

Proponents of this way of writing have dubbed this variant E-Prime. They argue that forcing the writer to think about how to eliminate the copula makes the resulting sentences more informative and less open to incorrect or ambiguous interpretation. You may be surprised to learn that several well known authors advocate E-Prime, and have released significant works written in this restricted form of English. Examples include the novels Under The Eye of God and A Covenant of Justice by science fiction author David Gerrold.

Although the inflections of the verb "to be" make it the verb with the most different forms in English, it pales next to some other common languages. For example, the equivalent verb "essere" in Italian (which means pretty much the same thing and tops the list of that language's most common verbs) inflects as follows:

Spanish possesses two different copulas: "ser" and "estar", which both inflect in multitudinous ways for various tenses and persons, giving the language over 100 different forms of copula to choose from. And in the other direction, Mandarin Chinese has no inflected forms of its copula verb at all.

[3] "Cool" is used here as an adjective describing something which is admired because of its interestingness, unusualness, aesthetic value, ethical value, or social cachet in the eyes of the person so describing it. The word is probably (I estimate) used more in this sense these days than in its original meaning of "slightly cold".

Some things which are cool: the Fonz, science fiction, roller coasters, webcomics, driving responsibly, that style of music you like, Dame Jane Goodall, bow ties.[7]